The term 'Enterprise software' has been abused (for long enough to confuse people) to sell things with the implied argument that 'you can't achieve your enterprise (i.e. b2b-level) software goals without this' usually with no justification for this even being offered.

The term is clearer when properly confined to its specific goals: It was coined in the late '80s at a time when someone had the idea that systems integration (hitherto occurring between departments of the same company) could be extended to include b2b-level systems integration. And that's all it is:

"enterprise software" is any software that provides or assists systems integration between several companies.